Degradation in Extrusion

Degradation during the extrusion process will often be a combination of thermal, mechanical, and chemical degradation. Factors that are important in determining 

Presence of solvolytic agents, oxygen, or other degradation promoting agents 

Knowledge of the residence time distribution (RTD) of an extruder provides valuable information about the details of the conveying process in the machine. The RTD is directly determined by the velocity profiles in the machine. Thus, if the velocity profiles are known, the RTD can be calculated. Various workers have made theoretical calculations of the RTD in singie screw extruders [48-50]. Obviously, theoretical calculations of the RTD require knowledge of the velocity profiles in the machine. Thus, the predicted RTD is only as accurate as the velocity profiles that form the basis of the calculations. In single screw extruders, the velocity profiles can be determined reasonably well, although usually a substantial number of simplifying assumptions are made. In other screw extruders, e.g., twin screw extruders, calculation of velocity profiles is rather complex and thus prediction of the RTD more difficult. 

The RTD Is determined by measuring the output response of a change in input. This is referred to as the stimulus response method as discussed by Levenspiel [61] and Himmelblau and Bischoff [62]. 

The system is disturbed by a stimulus and the response of the system to the stimulus is measured. Two common stimulus response techniques are the step input response and the pulse input response see Fig. 11.20. 

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